Semiconductor laser mounting with intact diffusion barrier layer

ABSTRACT

A first contact surface of a semiconductor laser chip can be formed to a target surface roughness selected to have a maximum peak to valley height that is substantially smaller than a barrier layer thickness. A barrier layer that includes a non-metallic, electrically-conducting compound and that has the barrier layer thickness can be applied to the first contact surface, and the semiconductor laser chip can be soldered to a carrier mounting along the first contact surface using a solder composition by heating the soldering composition to less than a threshold temperature at which dissolution of the barrier layer into the soldering composition occurs. Related systems, methods, articles of manufacture, and the like are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.14/873,080 filed Oct. 1, 2015 and entitled Semiconductor Laser MountingWith Intact Diffusion Barrier Layer, which is a Divisional of U.S.patent application Ser. No. 13/212,085, filed on Aug. 17, 2011 andentitled “Semiconductor Laser Mounting With Intact Diffusion BarrierLayer” which is related to co-owned U.S. patent application Ser. No.13/026,921, filed on Feb. 14, 2011 and entitled “Spectrometer withValidation Cell,” now issued as U.S. Pat. No. 8,358,417 on Jan. 22, 2013and also to co-owned U.S. patent application Ser. No. 13/027,000, filedon Feb. 14, 2011, and entitled “Validation and Correction ofSpectrometer Performance Using a Validation Cell,” now issued as U.S.Pat. No. 8,953,165 on Feb. 10, 2015. The disclosure of each applicationidentified in this paragraph is incorporated by reference herein in itsentirety.

TECHNICAL FIELD

The subject matter described herein relates to frequency stabilizationof semiconductor lasers, in particular to mounting techniques for suchlasers.

BACKGROUND

A tunable laser-based trace gas analyzer, such as for example a tunablediode laser absorption spectrometer (TDLAS) can employ a narrow linewidth (e.g. approximately single frequency) laser light source that istuned across a trace gas absorption frequency range of a target analytefor each measurement of a sample volume of gas. Ideally, the laser lightsource in such an analyzer exhibits no material change in the startingand ending frequency of successive laser scans under a constant laserinjection current and operating temperature. Additionally, long termstability of the frequency tuning rate of the laser as a function of thelaser injection current, over the scan range, and over repetitive scansover a prolonged period of service can also be desirable.

Depending on the operational wavelength, however, currently availabletunable laser sources (e.g. diode lasers and semiconductor lasers) cantypically exhibit a wavelength drift on the order of a few picometers(on the order of gigahertz) per day to fractions of picometers per day.A typical trace gas absorption band line width can in some instances beon the order of a fraction of a nanometer to microns. Thus, drift orother variations in the output intensity of the laser light source can,over time, introduce critical errors in identification andquantification of trace gas analytes, particularly in gas having one ormore background compounds whose absorption spectra might interfere withabsorption features of a target analyte.

SUMMARY

In one aspect, a method includes forming a first contact surface of asemiconductor laser chip to a target surface roughness selected to havea maximum peak to valley height that is substantially smaller than abarrier layer thickness of a barrier layer (e.g. a diffusion barrierlayer) to be applied to the first contact surface. The barrier layer,which includes a non-metallic, electrically conducting compound, is thenapplied to the first contact surface at that barrier layer thickness.The semiconductor laser chip is soldered to a carrier mounting using asolder composition. The soldering includes melting the solderingcomposition by heating the soldering composition to less than athreshold temperature at which dissolution of the barrier layer into thesoldering composition occurs.

In an interrelated aspect, an article of manufacture includes a firstcontact surface of a semiconductor laser chip formed to a target surfaceroughness. The target surface roughness includes a maximum peak tovalley height that is substantially smaller than a barrier layerthickness. The article of manufacture also includes a barrier layerhaving the barrier layer thickness applied to the first contact surfaceand a carrier mounting to which the semiconductor laser chip is solderedusing a solder composition. The barrier layer includes a non-metallic,electrically conducting compound. The semiconductor laser chip issoldered to the carrier mounting along the first contact surface by asoldering process that includes melting the soldering composition byheating the soldering composition to less than a threshold temperatureat which dissolution of the barrier layer into the soldering compositionoccurs.

In some variations one or more of the following features can optionallybe included in any feasible combination. The barrier layer can remaincontiguous subsequent to the soldering process such that no directcontact occurs between the solder composition and the materials of thesemiconductor laser chip and/or such that no direct path exists by whichconstituents of any of the semiconductor laser chip, the soldercomposition, and the carrier mounting can diffuse across the barrierlayer. Also subsequent to the soldering process, the solder compositioncan be characterized by substantially temporally stable electrical andthermal conductivities. In some examples, the solder composition used inthe soldering process can be provided as a solder preform that issubstantially non-oxidized. In other examples depositing the soldercomposition onto the heat sink or other carrier mounting, for example byevaporation, sputtering, or the like, can form a substantiallynon-oxidized solder composition. Additionally or alternatively, thesoldering process can further include performing the melting of thesoldering composition under a non-oxidizing or alternatively under areducing atmosphere.

The threshold temperature can in some implementations be less thanapproximately 400° C., or optionally less than 370° C., or optionallyless than 340° C., for example for solder compositions including but notlimited to one or more of gold germanium (AuGe), gold silicon (AuSi),gold tin (AuSn), silver tin (AgSn), silver tin copper (AgSnCu), silvertin lead (AgSnPb), silver tin lead indium (AgSnPbIn), silver tinantimony (AgSnSb), tin lead (SnPb), lead (Pb), silver (Ag), silicon(Si), germanium (Ge), tin (Sn), antimony (Sb), bismuth (Bi), indium(In), and copper (Cu). Some non-limiting examples of solder compositionsthat may be compatible with implementations of the current subjectmatter are also listed below.

The forming of the first contact surface can include polishing the firstcontact surface to achieve the target surface roughness prior toapplying the barrier layer. The target surface roughness can be lessthan approximately 100 Å rms or, alternatively, less than approximately40 Å rms. A first thermal expansion characteristic of the carriermounting can be matched to a second thermal expansion characteristic ofthe semiconductor laser chip. A metallization layer can be applied tothe first contact surface prior to applying the at least one barrierlayer, and a solder preparation layer can be applied to the firstcontact surface subsequent to applying the barrier layer and prior tothe soldering process. The metallization layer can optionally includeapproximately 600 Å thickness of titanium, the barrier layer canoptionally include approximately 1200 Å thickness of one or more oftitanium nitride (TiN_(X)), titanium oxy-nitride (TiN_(X)O_(Y)), ceriumgadolinium oxy-nitride (CeGdO_(Y)N_(X)), cerium oxide (CeO₂) tungstennitride (WN_(x)), and/or another non-metallic, electrically-conductingcompound; and the solder preparation layer can optionally includeapproximately 2000 to 5000 Å thickness of gold. In anotherimplementation, the barrier layer can optionally include a secondmetallic barrier layer including but not limited to platinum (Pt),palladium (Pd), nickel (Ni), tungsten (W), molybdenum (Mo) and metals:and the solder preparation layer can optionally contain approximately2000 to 5000 Å of gold (Au). Furthermore, the metallic barrier layer canbe applied directly to the first metallization layer and thenon-metallic barrier layer can be applied to the metallic barrier layer.In an alternative implementation the non-metallic barrier layer can beapplied to the first metallization layer and the metallic barrier layercan be applied to the non-metallic barrier layer.

A solder facilitation layer can optionally be added between the firstcontact surface and a second contact surface on the carrier mountingprior to the soldering process. The solder facilitation layer canoptionally include a metal that is not a component of a solderpreparation layer on either of the first contact surface or on a secondcontact surface of the carrier mounting. In this context, the term“solder preparation layer” is understood to refer to a topmost layer oneither or both of the first contact surface and the second contactsurface prior to the addition of the solder facilitation layer. Invarious optional variations, the solder preparation layer can be abarrier layer, a metallization layer, or some other layer. The adding ofthe solder facilitation layer can optionally include at least one ofplacing a sheet of the metal that is not a component of the solderpreparation layer between the first contact surface and the secondcontact surface prior to the soldering, and depositing a layer of themetal that is not a component of the solder preparation layer onto oneor both of the first contact surface and the second contact surfaceprior to the soldering.

An apparatus, which can in some implementations be a tunable diode laserabsorption spectrometer, can further include a light source thatincludes the carrier mounting and the semiconductor laser chip, adetector that quantifies a received intensity of light emitted from thelight source along a path length, at least one of a sample cell and afree space volume through which the path length passes at least once,and at least one processor that performs operations comprisingcontrolling a driving current to the laser source and receivingintensity data from the detector. The carrier mounting can includeand/or act as a heat spreader, heat sink, or the like. The at least oneprocessor can optionally cause the laser source to provide light havinga wavelength modulation frequency and can demodulate the intensity datareceived from the detector to perform a harmonic spectroscopy analysismethod. The at least one processor can mathematically correct ameasurement spectrum to account for absorption by compounds in a samplegas through which the path length passes. In some examples, themathematical correction can include subtraction of a reference spectrumfrom the measurement spectrum where the reference spectrum is collectedfor a sample of the sample gas in which a concentration of a targetanalyte has been reduced.

Systems and methods consistent with this approach are described as wellas articles that comprise a tangibly embodied machine-readable mediumoperable to cause one or more machines (e.g., computers, etc.) to resultin operations described herein. Similarly, computer systems are alsodescribed that may include a processor and a memory coupled to theprocessor. The memory may include one or more programs that cause theprocessor to perform one or more of the operations described herein.

The current subject matter may, in some implementations, provide one ormore advantages. For example, because soldering temperatures can beelevated without causing excessive damage to a diffusion barrier layer,the use of lower temperature, lead-based solders can be avoided. Thisfeature can enhance compliance with the Restriction of HazardousSubstances Directive (promulgated in the European Union) and othersimilar regulations, advisories, or the like regarding minimization ofthe use of lead. Solder compositions containing gold also generallyprovide better solder joint contiguity and surface wetting when goldplated surfaces are being joined. Because gold-containing solderstypically require higher soldering temperatures, the use of a hightemperature barrier layer that can survive such conditions without beingsubstantially degraded can be quite beneficial.

The details of one or more variations of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, show certain aspects of the subject matterdisclosed herein and, together with the description, help explain one ormore features or the principles associated with the disclosedimplementations. In the drawings,

FIG. 1 is a graph illustrating effects of laser drift on performance ofa laser absorption spectrometer;

FIG. 2 is a second graph illustrating additional effects of laser drifton performance of a laser absorption spectrometer;

FIG. 3 is a schematic diagram illustrating a semiconductor laser chipsecured to a carrier mount;

FIG. 4 is a process flow diagram illustrating aspects of a method havingone or more features consistent with implementations of the currentsubject matter;

FIG. 5 is a diagram showing an end elevation view of a conventionalTO-can mount such as are typically used for mounting semiconductor laserchips;

FIG. 6 is a diagram showing a magnified view of a carrier mount and asemiconductor laser chip affixed thereto;

FIG. 7 is a scanning electron micrograph showing a solder joint betweena semiconductor laser chip and a carrier mount;

FIG. 8 is a chart showing a phosphorous concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 9 is a chart showing a nickel concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 10 is a chart showing an indium concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 11 is a chart showing a tin concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 12 is a chart showing a lead concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 13 is a chart showing a tungsten concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 14 is a chart showing a gold concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7; and

When practical, similar reference numbers denote similar structures,features, or elements.

DETAILED DESCRIPTION

Experimental data have revealed that laser emission wavelength changesas small as 1 picometer (pm) or less between spectral scans in a laserabsorption spectrometer using a scannable or tunable laser source canmaterially alter a trace gas concentration determination with respect toa measurements obtainable with a spectral analyzer in its originalcalibration state. An example of spectral laser spectroscopy using adifferential spectroscopy approach is described in co-owned U.S. Pat.No. 7,704,301, the disclosure of which is incorporated herein in itsentirety. Other experimental data have indicated that a tunable diodelaser-based analyzer designed for low analyte concentration detectionand quantification (e.g. on the order of parts per million (ppm) ofhydrogen sulfide (H₂S) in natural gas) and employing a harmonic (e.g.2f) wavelength modulation spectral subtraction approach can unacceptablydeviate from its calibration state due to a shift in laser output of assmall as 20 picometers (pm) at constant injection current and constanttemperature (e.g. as controlled by a thermoelectric cooler).

In general terms, a laser frequency shift that can be acceptable formaintaining an analyzer calibration within its accuracy specificationdrops with smaller target analyte concentrations and also withincreasing spectral interferences from other components of a samplemixture at the location of the target analyte absorption. Formeasurements of higher levels of a target analyte in a substantiallynon-absorbing background, larger laser frequency shifts can be toleratedwhile maintaining the analyzer calibration state.

The graphs 100 and 200 shown in FIG. 1 and FIG. 2, respectively, showexperimental data illustrating potential negative impacts of laseroutput variations that may be caused by changes in characteristics (e.g.physical, chemical, and the like) of a semiconductor laser source overtime. The reference curve 102 shown in the graph 100 of FIG. 1 wasobtained with a tunable diode laser spectrometer for a reference gasmixture containing approximately 25% ethane and 75% ethylene. The testcurve 104 was obtained using the same spectrometer after some time hadpassed for a test gas mixture containing 1 ppm acetylene in a backgroundof approximately 25% ethane and 75% ethylene. Acetylene has a spectralabsorption feature in the range of about 300 to 400 on the wavelengthaxis of the chart 100 in FIG. 1. If the spectrometer were not adjustedin some manner to compensate for the drift observed in the test curve104 relative to the reference curve 102, the measured concentration ofacetylene from the spectrometer would be, for example, −0.29 ppm insteadof the correct value of 1 ppm.

Similarly, in FIG. 2, the chart 200 shows a reference curve 202 obtainedwith a tunable diode laser spectrometer for a reference gas mixturecontaining approximately 25% ethane and 75% ethylene. The test curve 204was obtained for a test gas mixture containing 1 ppm acetylene in abackground of approximately 25% ethane and 75% ethylene. As shown inFIG. 2, the line shape of the test curve 204 is distorted relative tothe line shape of the reference curve 202 due to drift or other factorsaffecting performance of the laser absorption spectrometer over time. Ifthe test curve 204 were not corrected to compensate for the distortionobserved in the test curve 204 relative to the reference curve 202, themeasured concentration of acetylene in the test gas mixture determinedby the spectrometer would be, for example, 1.81 ppm instead of the trueconcentration of 1 ppm.

Based on Ohm's Law (i.e. P=I²R where P is the power, I is the current,and R is the resistance), a current-driven semiconductor laser chip willgenerate waste heat that increases approximately as the square of theinjection current driving the laser. While the resistance, R, of thesemiconductor diode laser assembly is not typically linear or constantwith changes in temperature, an approximately quadratic increase inwaste heat with increases in current is generally representative ofreal-world performance. Thermal roll-over, in which the power output ofa laser is reduced at excessive temperatures, can typically occurbecause the lasing efficiency of a typical band-gap type directsemiconductor laser diode decreases with increasing p-n junctionoperating temperature. This is especially true for infrared lasers, suchas for example lasers based on indium phosphide (InP) or galliumantimonide (GaSb) material systems.

Single frequency operation of an infrared semiconductor laser can beachieved by employing DFB (distributed feedback) schemes, whichtypically use optical gratings, either incorporated in the laser ridgeof the semiconductor laser crystal in the form of semiconductor crystalindex of refraction periodicities or placed laterally to the laser ridgeas metal bars. The effective optical periods of the approaches of thevarious gratings determining the laser emission wavelength can typicallydepend upon the physical spacing of the metal bars of the grating orupon the physical dimension of the ridge-regrown semiconductor materialzones with different index of refraction and the index of refraction ofthe respective semiconductor materials. In other words, the emissionwavelength of a semiconductor laser diode, such as are typically usedfor tunable diode laser spectroscopy, can depend primarily upon thelaser p-n junction and on the laser crystal operating temperature andsecondarily on the carrier density inside the laser. The laser crystaltemperature can change the grating period as a function of thetemperature dependent thermal expansion of the laser crystal along itslong optical cavity axis and as a function of the temperature dependentindex of refraction.

Typical injection current-related and temperature-related wavelengthtuning rates of infrared lasers useable for tunable diode laser tracegas analyzers can be on the order of approximately 0.1 nanometers per °C. and approximately 0.1 nanometers per milli-ampere. As such, it can bedesirable to maintain semiconductor laser diodes for precision TDLASdevices at a constant operating temperature within a few thousandths ofa ° C. and at injection currents that are controlled to within a fewnano-amperes.

Long term maintenance and regeneration of a TDLAS calibration state andthe related long term measurement fidelity with respect to the originalinstrument calibration can require the ability to substantiallyreplicate the correct laser operating parameters in the wavelength spacefor any given measurement. This can be desirable for spectroscopytechniques employing subtraction of spectral traces (e.g. differentialspectroscopy), such as is described in co-owned U.S. Pat. No. 7,704,301;pending U.S. patent application Ser. No. 13/027,000 and Ser. No.13/026,091 and Ser. No. 12/814,315; and U.S. Provisional Application No.61/405,589, the disclosures of which are incorporated by referenceherein.

Commercially available single frequency semiconductor lasers that aresuitable for trace gas spectroscopy in the 700 nm to 3000 nm spectralrange have been found to generally exhibit a drift of their centerfrequency over time. Drift rates of several picometers (pm) to fractionsof a pm per day have been confirmed by performing actual molecular tracegas spectroscopy over periods of 10 days to more than 100 days. Lasersthat may behave as described can include, but are not limited to, laserslimited to single frequency operation by gratings etched into the laserridge (e.g. conventional telecommunications grade lasers), Bragggratings (e.g. vertical cavity surface emitting lasers or VCSELs),multiple layer narrow band dielectric mirrors, laterally coupledgratings, and the like. Frequency drift behavior has been observed withsemiconductor diode lasers; VCSELs; horizontal cavity surface emittinglasers HCSEL's (HCSELs); quantum cascade lasers built on semiconductormaterials including but not limited to indium phosphide (InP), galliumarsenide (GaAs), gallium antimonide (GaSb), gallium nitride (GaN),indium gallium arsenic phosphide (InGaAsP), indium gallium phosphide(InGaP), indium gallium nitride (InGaN), indium gallium arsenide(InGaAs), indium gallium aluminum phosphide (InGaAlP), indium aluminumgallium arsenide (InAlGaAs), indium gallium arsenide (InGaAs), and othersingle and multiple quantum well structures.

Approaches have been previously described to re-validate the performanceof a tunable laser. For example, as described in U.S. patent applicationSer. No. 13/026,921 and Ser. No. 13/027,000 referenced above, areference absorption line shape collected during a calibrated state ofan analyzer can be compared to a test absorption line shape collectedsubsequently. One or more operating parameters of the analyzer can beadjusted to cause the test absorption line shape to more closelyresemble the reference absorption line shape.

Reduction of the underlying causes of frequency instability insemiconductor-based tunable lasers can also be desirable, at leastbecause compensation of laser shift and outputted line shapes tomaintain analyzer calibration by adjusting the semiconductor diode laseroperating temperature or the median drive current may only be possibleover limited wavelength shifts due to a typically non-linear correlationbetween injection current and frequency shift in semiconductor laserdiodes (e.g. because of thermal roll-over as discussed above). Thenonlinearity of the frequency shift as a function of injection currentmay change as a function of laser operating temperature set by atemperature control device (e.g. a thermoelectric cooler or TEC) and themedian injection current. At higher control temperatures, thermalroll-over may occur at lower injection currents while at lower controltemperatures, the roll-over may occur at higher injection currents.Because the control temperature and injection current combined determinethe laser emission wavelength, not all combinations of controltemperature and median injection current used to adjust the laserwavelength to the required target analyte absorption line will providethe same frequency scan and absorption spectra.

Accordingly, one or more implementations of the current subject matterrelate to methods, systems, articles or manufacture, and the like thatcan, among other possible advantages, provide semiconductor-based lasersthat have substantially improved wavelength stability characteristicsdue to a more temporally stable chemical composition of materials usedin affixing a semiconductor laser chip to a mounting device. Someimplementations of the current subject matter can provide or include asubstantially contiguous and intact diffusion barrier layer thatincludes at least one non-metallic layer and alternatively at least onenon-metallic and at least one metallic barrier layer at or near acontact surface between a semiconductor laser chip and a mountingsurface. Drift of single frequency lasers can be reduced or evenminimized, according to one or more implementations, by employingsemiconductor laser designs, laser processing, electrical connections,and heat sinking features that reduce changes in heat conductivity, instress and strain on the active laser, and in electrical resistivity ofthe injection current path over time.

FIG. 3 illustrates an example of an apparatus 300 including asemiconductor laser chip 302 affixed to a mounting device 304 by a layerof solder 306 interposed between a contact surface 310 of thesemiconductor laser chip 302 and the mounting device 304. The mountingdevice can function as a heat sink and can provide one or moreelectrical connections to the semiconductor laser chip 302. One or moreother electrical connections 312 can be provided to connect a p or njunction of the semiconductor laser chip 302 to a first polarity and theother junction to a second polarity, for example via conduction throughthe solder layer 306 into the carrier mount 304.

FIG. 4 shows a process flow chart illustrating a method includingfeatures that can be present in one or more implementations of thecurrent subject matter. At 402, a first contact surface of asemiconductor laser chip is formed to a target surface roughness. Thetarget surface roughness is selected to have a maximum peak to valleyheight that is substantially smaller than a barrier layer thickness of abarrier layer to be applied to the first contact surface. At 404, thatbarrier layer is applied to the first contact surface with the barrierlayer thickness. The barrier layer includes the at least onenon-metallic, electrically-conducting compound, examples of whichinclude but are not limited to titanium nitride (TiN_(X)), titaniumoxy-nitride (TiN_(X)O_(Y)), cerium gadolinium oxy-nitride(CeGdO_(y)N_(X)) cerium oxide (CeO₂), and tungsten nitride (WN_(x)). At406, the semiconductor laser chip is soldered to a carrier mountingalong the first contact surface using a solder composition. Thesoldering includes melting the soldering composition by heating thesoldering composition to less than a threshold temperature at whichdissolution of the barrier layer into the soldering composition occurs.

In some implementations, a contact surface 310 of a laser semiconductorchip 302 can be polished or otherwise prepared to have a target surfaceroughness of less than approximately 100 Å rms, or alternatively of lessthan approximately 40 Å rms. Conventional approaches have typically notfocused on the surface roughness of the contact surface 310 and haveconsequently had surface roughness values of greater than approximately1 μm rms. Subsequent to preparing a sufficiently smooth contact surface310, the contact surface 310 can be treated to form one or more barrierlayers.

Creation of a barrier layer that can survive the soldering process canbe aided by polishing of the first contact surface 310 to a low surfaceroughness. In general, a total thickness of a metallic barrier layer,for example one made of platinum, may only be deposited at a limitedthickness due to very high stresses that can lead to a separation ofthicker layers from the semiconductor material of the semiconductorlaser chip 302. The barrier layer can include multiple layers ofdiffering materials. In an implementation, at least one of the barrierlayers can include a non-metallic, electrically conducting compound,such as for example titanium nitride (TiN_(X)), titanium oxy-nitride(TiN_(X)O_(Y)), cerium gadolinium oxy-nitride (CeGd_(y)ON_(X)), ceriumoxide (CeO₂), and tungsten nitride (WN_(x)). One or more additionalbarrier layers overlaying or underlaying the first barrier layer caninclude a metal including but not limited to platinum (Pt), palladium(Pd), nickel (Ni), tungsten (W), molybdenum (Mo) titanium (Ti), tantalum(Ta), zirconium (Zr), cerium (Ce), gadolinium (Gd), chromium (Cr),manganese (Mn), aluminum (Al), beryllium (Be), and Yttrium (Y).

A solder composition can in some implementations be selected from acomposition having a liquidus temperature, i.e. the maximum temperatureat which solid crystals of an alloy can co-exist with the melt inthermodynamic equilibrium, of less than approximately 400° C., oroptionally of less than approximately 370° C., or optionally of lessthan approximately 340° C. Examples of solder compositions consistentwith one or more implementations of the current subject matter caninclude, but are not limited to gold germanium (AuGe), gold silicon(AuSi), gold tin (AuSn), silver tin (AgSn), silver tin copper (AgSnCu),silver tin lead (AgSnPb), silver tin lead indium (AgSnPbIn), silver tinantimony (AgSnSb), tin lead (SnPb), and lead (Pb). Examples of specificsolder compositions that are consistent with one or more implementationsof the current subject matter include, but are not limited to thefollowing: approximately 48% Sn and approximately 52% In; approximately3% Ag and approximately 97% In; approximately 58% Sn and approximately42% In; approximately 5% Ag, approximately 15% Pb, and approximately 80%In; approximately 100% In; approximately 30% Pb and approximately 70%In; approximately 2% Ag, approximately 36% Pb, and approximately 62% Sn;approximately 37.5% Pb, approximately 37.5% Sn, and approximately 25%In; approximately 37% Pb and approximately 63% Sn; approximately 40% Pband approximately 60% In; approximately 30% Pb and approximately 70% Sn;approximately 2.8% Ag, approximately 77.2% Sn, and approximately 20% In;approximately 40% Pb and approximately 60% Sn; approximately 20% Pb andapproximately 80% Sn; approximately 45% Pb and approximately 55% Sn;approximately 15% Pb and approximately 85% Sn; approximately 50% Pb andapproximately 50% In; approximately 10% Pb and approximately 90% Sn;approximately 10% Au and approximately 90% Sn; approximately 3.5% Ag andapproximately 96.5% Sn; approximately 60% Pb and approximately 40% In;approximately 3.5% Ag, approximately 95% Sn, and approximately 1.5% Sb;approximately 2.5% Ag and approximately 97.5% Sn; approximately 100% Sn;approximately 99% Sn and approximately 1% Sb; approximately 60% Pb andapproximately 40% Sn; approximately 97% Sn and approximately 3% Sb;approximately 95% Sn and approximately 5% Sb; approximately 63.2% Pb,approximately 35% Sn, and approximately 1.8% In; approximately 70% Pband approximately 30% Sn; approximately 75% Pb and approximately 25% In;approximately 80% Pb approximately 20% Sn; approximately 81% Pb andapproximately 19% In; approximately 80% Au and approximately 20% Sn;approximately 86% Pb, approximately 8% Bi, approximately 4% Sn, andapproximately 1% In, approximately 1% Ag; approximately 85% Pb andapproximately 15% Sn; approximately 2% Ag, approximately 88% Pb, andapproximately 10% Sn; approximately 5% Ag, approximately 90% Pb, andapproximately 5% Sn; approximately 95% Pb and approximately 5% Sb;approximately 2.5% Ag, approximately 92.5% Pb, and approximately 5% Sn;approximately 2.5% Ag, approximately 92.5% Pb, and approximately 5% In;approximately 90% Pb and approximately 10% Sn; approximately 2.5% Ag andapproximately 97.5% Pb; approximately 2.5% Ag; approximately 95.5% Pb,and approximately 2% Sn; approximately 78% Au and approximately 22% Sn;approximately 1.5% Ag, approximately 97.5% Pb, and approximately 1% Sn;approximately 5% Ag, approximately 90% Pb, and approximately 5% In;approximately 95% Pb and approximately 5% In; and approximately 95% Pband approximately 5% Sn.

FIG. 5 shows an end elevation view of a conventional transistor outlinecan (TO-can) mount 500 such as is typically used in mounting ofsemiconductor laser chips for use in telecommunications applications.TO-cans are widely used electronics and optics packaging platforms usedfor mechanically mounting, electrically connecting, and heat sinkingsemiconductor chips such as lasers and transistors and are available ina variety of different sizes and configurations. An outer body 502encloses a post or heat sink member 504 which can be made of metal, suchas for example a copper tungsten sintered metal, copper-diamond sinteredmetal, or iron-nickel alloys including Kovar, alloy 42, and alloy 52.Two insulated electrical pass-throughs 506 can be included to provideelectrical contacts for connection to the p and n junctions on asemiconductor laser chip 302. The semiconductor laser chip 302 can bemounted to a carrier sub-mount, which can in some examples be formed ofsilicon. As noted above, the semiconductor laser chip 302 can be joinedto the carrier mount 304 (also referred to as a carrier mounting) by alayer of solder 306, which is not shown in FIG. 5 due to scaleconstraints. FIG. 6 shows a magnified view 600 of the post or heat sinkmember 504, the carrier mount 304, the semiconductor laser chip 302, andthe solder 306 joining the semiconductor laser chip 302 to the carriermount. The carrier mount 304 can in turn be soldered to the post or heatsink member 504 by a second solder layer 602.

According to one or more implementations of the current subject matter,mono-component layers (or surfaces) of a material dissimilar from thesolder preparation layer, which includes but is not limited to gold, canserve similarly as solder alloys, enabling low temperature joining oftwo gold surfaces for instance. In joining metal components, this iscommonly referred to as liquidus or liquid diffusion bonding since itapparently creates a very thin liquid interface layer between certaindissimilar metals which are brought in physical contact under elevatedtemperature. The temperature necessary to cause this effect to occur istypically significantly lower than the component melting temperatures.Once the initial joining has taken place, thermal separation cantypically require quite high temperatures, approaching or reaching thecomponent melting temperatures. In one example, contact between a silversurface and a gold surface can result in a hermetic joint at atemperature of approximately 150° C. to approximately 400° C., which issignificantly lower than the separate melting temperatures of silver(950° C.) and gold (1064° C.). In another example, copper oxide canserve as a bonding promotion layer which can reduce a joiningtemperature between two metal surfaces significantly below the metalmelting points.

Thus, in some implementations, solder compositions including lead (Pb),silver (Ag), silicon (Si), germanium (Ge), tin (Sn), antimony (Sb),bismuth (Bi), indium (In), and copper (Cu) as well as those discussedelsewhere herein can be used in association with deposition of one ormore solder-facilitating mono-component material layers or thin sheets(e.g. preforms) on the first contact surface 310 and/or second contactsurface 314 prior to the soldering process. The one or moresolder-facilitating mono-component material layers or thin sheets can bedissimilar from other barrier and/or metallization layers on the firstcontact surface 310 and/or second contact surface 314. One example of amethod for applying solder-facilitating mono-component material layersor thin sheets can include depositing a thin layer of a metal differingfrom those metals present in the solder preparation layer on top of thebarrier and/or metallization layers on the first contact surface 310and/or second contact surface 314. Such a thin layer can be evaporatedor otherwise deposited onto one or both of the first contact surface 310and the second contact surface 314 shortly before the heat assistedjoining process takes place, in order to prevent or minimize oxidation.Alternately, a thin sheet of a metal dissimilar from the solderpreparation layer can be placed between the semiconductor laser chip 302and the mounting device 304. The soldering process can then proceed asdiscussed above.

FIG. 7 shows an electron micrograph 700 showing a highly magnifiedsolder layer 306 interposed between a semiconductor laser chip 302 and acarrier mount 304. A second barrier layer 702 of nickel is also providedon the second contact surface 704 of the carrier mount 304. A verticalaxis 706 is displayed atop the electron micrograph to delineate distancefrom an arbitrarily chosen origin coordinate (marked as “0” on the axis706) to a linear distance of 50 microns away (marked as “50” on the axis706). The semiconductor laser chip 302 shown in FIG. 7 was not preparedwith a smooth first contact surface 310 as described herein consistentaccording various implementations of the current subject matter. As aresult, the first contact surface 310 exhibits substantial surfaceroughness, and no contiguous barrier layer remains to separate thematerial of the semiconductor laser chip 302 from the solder after thesoldering process. FIG. 8 through FIG. 14 show a series of charts 800,900, 1000, 1100, 1200, 1300, and 1400 showing relative concentrations ofphosphorous, nickel, indium, tin, lead, tungsten, and gold,respectively, as a function of distance along the axis 706 in FIG. 7.The relative concentrations were determined by an X-ray diffractiontechnique.

As shown in the chart 800 of FIG. 8, a large phosphorous concentrationis observed in the semiconductor laser chip 302 (distance greater thanabout 36 μm) due to the semiconductor laser chip 302 being a crystal ofindium phosphide (InP). Additional high relative concentrations ofphosphorous are observed in the nickel barrier layer 702, which isactually formed of a first layer 710 of nickel deposited by anelectroless process that incorporates some phosphorous into thedeposited nickel and a second layer of nickel deposited by anelectrolytic process that incorporates less or no phosphorous into thedeposited nickel. A non-zero concentration of phosphorous occurs both inthe solder (which is composed of a tin-lead alloy and does not containany phosphorus in its original state) and in the electrolytic secondlayer 712 of nickel. These non-zero concentrations are respectively dueto diffusion of phosphorous from the crystal structure of thesemiconductor laser chip 302 and from the electroless first layer 710 ofnickel.

FIG. 9 illustrates that some nickel also diffuses into the solder 306from the nickel layer 702 and further into the crystal structure of thesemiconductor laser chip 302. Similarly, indium diffuses into the solder306 and from there into the carrier mount across the nickel barrierlayer 702 as shown in the chart 1000 of FIG. 10. Tin, which is a primarycomponent of the solder 306, does not remain in the solder 306, but alsodiffuses into the crystal structure of the semiconductor laser chip 302as shown in the chat 1100 of FIG. 11. Lead also diffuses out of thesolder layer 306 as shown in the chart 1200 of FIG. 12, but to a lesserdegree than does the tin from the solder 306. Tungsten from thetungsten-copper carrier mount 304 and gold from solder preparationlayers deposited on both of the first contact surface 310 and the secondcontact surface 702 diffuse into the solder and to a small extent intothe semiconductor laser chip 302 as shown in the charts 1300 and 1400 ofFIG. 13 and FIG. 14.

Accordingly, features of the current subject matter that allow themaintenance of a contiguous, intact barrier layer at least at the firstcontact surface 310 of the semiconductor laser chip 302, and alsodesirably at the second contact surface 704 of the carrier mount 304 canbe advantageous in minimizing diffusion of elements from the carriermount and/or semiconductor laser chip across the barrier layer and canthereby aid in maintaining a more temporally consistent composition ofboth the solder layer 306 and the crystal structure of the semiconductorlaser chip 302. The presence of phosphorous and/or other reactivecompounds or elements, such as for example oxygen, antimony, silicon,iron and the like in the solder layer 306 can increase a tendency of thesolder alloy components to react and thereby change in chemicalcomposition, in crystal structure, hermeticity and, more importantly, inelectrical and/or thermal conductivity. Such changes can lead toalteration in the laser emission characteristics of a semiconductorlaser chip 302 in contact with the solder layer 306.

Furthermore, diffusion of solder components, such as for example lead;silver; tin; and the like; and/or carrier mount components such astungsten, nickel, iron, copper and the like, into the crystal structureof the semiconductor laser chip 302 can also cause changes in the laseremission characteristics over time.

Implementations of the current subject matter can provide one or moreadvantages, including but not limited to maintaining a contiguousdiffusion barrier layer between a laser crystal or other semiconductorchip and its physical mounting, preventing inter-diffusion of soldercompounds into the laser crystal and vice versa, and preventingcontamination of the solder. Inter-diffusion and/or electro-migrationhave been found to cause changes in the electrical resistivity, and to alesser extent the heat conduction properties, of the contact. Very smallchanges in resistive heating of even one of the electrical contactsproviding a driving current to a semiconductor laser chip can lead tofrequency changes in the light produced by the semiconductor laser chip.

In some observed examples using conventional semiconductor laser chipmounting approaches, induced shifts in the laser output can be greaterthan a picometer per day. Implementations of the current subject mattercan therefore include one or more techniques for improving barrierlayers at one or more of the first contact surface 310 between thesolder layer 306 and the semiconductor laser chip 302 and the secondcontact layer 702 between the solder layer 306 and the carrier mount304. In one example, an improved barrier layer at the second contactsurface 702 can include an electroless plated nickel underlayer 710, forexample to preserve edge definition of a copper tungsten submount or thelike, covered by a minimum thickness of an electrolytic nickel layer 712as the final layer before deposition of a gold solder preparation layer.In another example, a single layer of a sputtered barrier material,including but not limited to at least one of nickel, platinum,palladium, and electrically conducting non-metallic barrier layers, canbe used as a single barrier layer at the first contact surface 310. Asoxidation of the solder material prior to soldering of the semiconductorlaser chip 302 to the carrier mount 304 can introduce oxygen and otherpotentially reactive contaminants, it can be advantageous to use solderforms that have not been allowed to substantially oxidize prior to use.Alternatively, the soldering process can be performed under anon-oxidizing atmosphere or under a reducing atmosphere including butnot limited to vacuum, pure nitrogen pure hydrogen gas (H₂), forming gas(approximately 5% hydrogen in 95% nitrogen), and formic acid in nitrogencarrier gas to remove or at least reduce the presence of oxidizedcompounds in the solder composition on the metalized semiconductorcontact surface and the carrier mounting surface.

Suitable barrier layers to be deposited on the first contact surface 310and/or the second contact surface 702 can include, but are not limitedto, platinum (Pt), palladium (Pd), nickel (Ni), titanium nitride(TiN_(X)), titanium oxy-nitride (TiN_(X)O_(Y)), tungsten nitride(WN_(x)), cerium oxide (CeO₂), and cerium gadolinium oxy-nitride(CeGdO_(Y)N_(X)). These compounds, as well as other comparable compoundsthat can be deposited by sputtering or vapor deposition onto the firstand/or second contact surfaces, can provide a barrier layer that has asufficiently high temperature resistance during the soldering process asto not dissolve in the solder or otherwise degrade sufficiently to causebreakdown of the barrier qualities necessary to prevent cross-barrierdiffusion of semiconductor laser materials into the solder or soldercomponents into the semiconductor laser crystal. The second barrierlayer 702 applied to the second contact surface 704 can in someimplementations include a sintered diamond-copper layer. A process forcreation of a non-metallic, electrically-conducting barrier layer 702can include first depositing titanium via a thin film depositionprocess, including but not limited to sputtering, electron beamevaporation, chemical vapor deposition, atomic layer deposition, and thelike, and then adding nitrogen to react with the deposited titanium. Inanother implementation, a first metallization layer can be deposited bya thin film deposition process, and nitrogen ions can be used forsputtering titanium, for example in a nitrogen gas background, to createthe non-metallic barrier layer. Chemical vapor deposition can also oralternatively be used to create non-metallic barrier layers. In anotherimplementation, gas phase reactions of the components elements orcompounds forming the non-metallic electrically conductive compound canbe used to create multi-component non-metallic barrier layers.

In some implementations, the heat conductivity of a carrier mount 304can advantageously exceed 50 Watts per meter-Kelvin or, optionally 100Watts per meter-Kelvin or, optionally 150 Watts per meter-Kelvin.Suitable carrier mount materials can include, but are not limited tocopper tungsten, tungsten, copper-diamond, aluminum nitride, silicon,silicon nitride, silicon carbide, beryllium oxide, alumina (Al₂O₃),Kovar, Alloy 42, Alloy 52, and the like. A heat spreader or carriermount 304 that is thermally expansion matched to the semiconductor laserchip 302 can be used in some implementations. In one example consistentwith an implementation of the current subject matter, an approximately15% copper, approximately 85% tungsten sintered metal heat spreader, aberyllium oxide heat spreader, an alumina heat spreader, a sapphire heatspreader, a copper-diamond heat spreader, or the like can provide a goodthermal expansion match to a gallium antimonide (GaSb) semiconductorlaser chip 302 at around approximately 7 ppm° C.⁻¹. In another exampleconsistent with an implementation of the current subject matter, a puretungsten heat spreader, a silicon heat spreader, a silicon nitride heatspreader, a silicon carbide heat spreader, a sapphire heat spreader, acopper diamond heat spreader, or an aluminum nitride (AlN) heat spreadercan be used as a carrier mount 304 to provide a good thermal expansionmatch to an indium phosphide (InP) semiconductor laser chip 302 ataround 4.5 ppm° C.⁻¹. A silicon, silicon carbide, silicon nitride,aluminum nitride, tungsten, copper diamond heat spreader, or the likecan also be used as the carrier sub-mount 304, for example for an indiumphosphide (InP) semiconductor laser chip 302.

Other carrier mounts consistent with implementations of the currentsubject matter include, but are not limited to shaped copper tungstenheat spreaders, including but not limited to semiconductor laserindustry standard c-mounts and CT-mounts, TO-cans, pattern metallizedceramics, pattern metallized silicon, pattern metallized siliconcarbide, pattern metallized silicon nitride, pattern metallizedberyllium oxide, pattern metallized alumina, pattern metallized aluminumnitride, copper-diamond, pure copper with one or more sections ofexpansion-matched submounts to match to one or more semiconductor laserchip compositions, tungsten submounts brazed into a copper or coppertungsten c-mount, or the like. Semiconductor laser chips 302 can beformed, without limitation of indium phosphide crystals, galliumarsenide crystals, gallium antimonide crystals, gallium nitridecrystals, and the like.

The subject matter described herein can be embodied in systems,apparatus, methods, and/or articles depending on the desiredconfiguration. The implementations set forth in the foregoingdescription do not represent all implementations consistent with thesubject matter described herein. Instead, they are merely some examplesconsistent with aspects related to the described subject matter.Although a few variations have been described in detail above, othermodifications or additions are possible. In particular, further featuresand/or variations can be provided in addition to those set forth herein.For example, the implementations described above can be directed tovarious combinations and subcombinations of the disclosed featuresand/or combinations and subcombinations of several further featuresdisclosed above. In addition, the logic flows depicted in theaccompanying figures and/or described herein do not necessarily requirethe particular order shown, or sequential order, to achieve desirableresults. Other implementations may be within the scope of the followingclaims.

What is claimed is:
 1. A method comprising: forming a first contactsurface of a semiconductor laser chip to a target surface roughness, thetarget surface roughness being selected to have a maximum peak to valleyheight that is substantially smaller than a barrier layer thickness, thesemiconductor laser chip comprising: a first junction connected to ap-type region of the semiconductor laser chip; and a second junctionconnected to an n-type region of the semiconductor laser chip, the firstcontact surface being closer to the p-type region of the semiconductorlaser chip than to the n-type region; applying a barrier layer havingthe barrier layer thickness to the first contact surface of thesemiconductor laser chip, the barrier layer comprising a non-metallic,electrically conducting compound, the forming of the first contactsurface comprises polishing the first contact surface to achieve thetarget surface roughness prior to applying the barrier layer; andsoldering the semiconductor laser chip along the barrier layer to acarrier mounting using a solder composition, the soldering comprisingmelting the soldering composition by heating the soldering compositionto less than a threshold temperature at which dissolution of the barrierlayer into the soldering composition occurs.
 2. The method as in claim1, wherein, subsequent to the soldering, the barrier layer remainscontiguous such that no direct contact occurs between semiconductormaterials of the semiconductor laser chip and the solder composition. 3.The method as in claim 1, wherein, subsequent to the soldering process,the barrier layer remains substantially contiguous such that no directpath exists by which constituents of any of the semiconductor laserchip, the solder composition, and the carrier mounting can diffuseacross the barrier layer.
 4. The method as in claim 1, wherein,subsequent to the soldering process, the solder composition ischaracterized by substantially temporally stable electrical and thermalconductivities.
 5. The method as in claim 4, further comprisingproviding the solder composition as at least one of a solder preformthat is substantially non-oxidized and a deposited layer that issubstantially non-oxidized.
 6. The method as in claim 4, wherein thesoldering further comprises performing the melting of the solderingcomposition under at least one of a reducing atmosphere and anon-oxidizing atmosphere.
 7. The method as in claim 1, wherein thethreshold temperature is less than approximately 400° C.
 8. The methodas in claim 1, wherein the threshold temperature is less thanapproximately 370° C.
 9. The method as in claim 1, wherein the thresholdtemperature is less than approximately 340° C.
 10. The method as inclaim 1, wherein the target surface roughness is less than approximately100 Å rms.
 11. The method as in claim 1, wherein the target surfaceroughness is less than approximately 40 Å rms.
 12. The method as inclaim 1, further comprising matching a first thermal expansioncharacteristic of the carrier mounting to a second thermal expansioncharacteristic of the semiconductor laser chip.
 13. The method as inclaim 1, further comprising: applying a metallization layer to the firstcontact surface prior to applying the barrier layer; and applying asolder preparation layer to the first contact surface subsequent toapplying the barrier layer and prior to the soldering.
 14. The method asin claim 13, wherein the metallization layer comprises approximately 600Å thickness of titanium, the barrier layer comprises approximately 1200Å thickness of one or more of platinum (Pt), palladium (Pd), nickel(Ni), tungsten (W), molybdenum (Mo), tantalum (Ta), zirconium (Zr),cerium (Ce), gadolinium (Gd), chromium (Cr), manganese (Mn), aluminum(Al), beryllium (Be), Yttrium (Y), titanium nitride (TiN_(X)), titaniumoxy-nitride (TiN_(X)O_(Y)), tungsten nitride (WN_(x)), cerium oxide(CeO₂), and cerium gadolinium oxy-nitride (CeGdON_(X)); and the solderpreparation layer comprises approximately 2000 to 5000 Å thickness ofgold.
 15. The method as in claim 1, further comprising applying a secondbarrier layer to a second contact surface of the carrier mounting, thesoldering of the semiconductor laser chip being performed along thesecond contact surface.
 16. The method as in claim 1, further comprisingadding a solder facilitation layer between the first contact surface anda second contact surface on the carrier mounting prior to the soldering,the solder facilitation layer comprising a metal that is not a componentof a solder preparation layer on either or both of the first contactsurface and a second contact surface.
 17. The method as in claim 16,wherein the adding of the solder facilitation layer comprises at leastone of placing a sheet of the metal between the first contact surfaceand the second contact surface prior to the soldering, and depositing alayer of the metal that is not a component of the solder compositiononto one or both of the first contact surface and the second contactsurface prior to the soldering.
 18. An article of manufacturecomprising: a first contact surface of a semiconductor laser chip formedand polished to achieve a target surface roughness, the target surfaceroughness having a maximum peak to valley height that is substantiallysmaller than a barrier layer thickness, the semiconductor laser chipcomprising: a first junction connected to a p-type region of thesemiconductor laser chip; and a second junction connected to an n-typeregion of the semiconductor laser chip, the first contact surface beingcloser to the p-type region of the semiconductor laser chip than to then-type region; a barrier layer having the barrier layer thicknessapplied to the first contact surface of the semiconductor laser chip,the barrier layer comprising a non-metallic, electrically-conductingcompound; and a carrier mounting to which the semiconductor laser chipis soldered along the barrier layer using a solder composition, thesemiconductor laser chip being soldered to the carrier mounting by asoldering process comprising melting the soldering composition byheating the soldering composition to less than a threshold temperatureat which dissolution of the barrier layer into the soldering compositionoccurs.
 19. The article of claim 18, further comprising a heat sink incontact with the carrier mounting, wherein the carrier mounting isbetween the heat sink and the semiconductor laser chip.
 20. The articleof claim 18, further comprising a heat sink and a second solder layerbetween the heat sink and the carrier mounting.